US President Donald Trump said he ordered a targeted military strike on the airfield in Syria from which a deadly chemical attack was launched and added there could be no dispute that Syria had used banned chemical weapons.

"It is in the vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons," Trump said.

"There can be no dispute that Syria used banned chemical weapons, violated its obligations under the chemical weapons convention and ignored the urging of the UN Security Council."

MC3 (SW) Robert S. Price

US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea.

The president also called on "civilised nations" to join the US in "seeking to end the slaughter and bloodshed in Syria". Trump said "peace and harmony will prevail" so long as the US continues to stand for justice.

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MC3 (SW) Robert S. Price

The missiles are fired.

The barrage of cruise missiles is the first direct American assault on the Syrian government and Donald Trump's most dramatic military order since becoming US president.

The surprise strike marked a striking reversal for Trump, who warned as a candidate against the US getting pulled into the Syrian civil war, now in its seventh year. But the president appeared moved by the photos of children killed in the chemical attack, calling it a "disgrace to humanity" that crossed "a lot of lines".

About 60 US Tomahawk missiles, fired from warships in the Mediterranean Sea, targeted an air base in retaliation for a chemical weapons attack that American officials believe Syrian government aircraft launched with a nerve agent, possibly sarin.

U.S. Navy

Guided missile destroyer USS Ross, pictured in 2014.

The Syrian government has denied it was behind the attack, telling state television it had been a target of "American aggression".

The target was identified as an airbase in Homs. A US official told Reuters that targets included an airstrip, aircraft and fuel stations, adding the missiles themselves struck their targets at 3.45am in Syria on Friday (12.45pm NZ time). Details on the results of the strikes were not immediately known. 

Trump did not announce the attacks in advance, though he and other national security officials ratcheted up their warnings to the Syrian government.

HANDOUT

Shayrat Airfield in Homs, Syria is seen in this DigitalGlobe satellite image released by the US after announcing the missile strikes.

"I think what happened in Syria is one of the truly egregious crimes and shouldn't have happened and it shouldn't be allowed to happen," Trump told reporters travelling on Air Force One to Florida, where he was holding a two-day summit with Chinese President Xi Jinping.

The strike came as Trump was hosting Xi in meetings focused in part on another pressing US security dilemma: North Korea's nuclear program. Trump's actions in Syria could signal to China that the new president isn't afraid of unilateral military steps, even if key nations like China are standing in the way.

AT ODDS WITH RUSSIA?

US military action in Syria potentially puts the US president at odds with Russia, which has air and ground forces in Syria after intervening there on Assad's side in 2015 and turning the tide against mostly Sunni Muslim rebel groups.

Trump has until now focused his Syria policy almost exclusively on defeating Islamic State militants in northern Syria, where US special forces are supporting Arab and Kurdish armed groups.

The US military gave Russian forces advanced notice of its strikes on a Syrian airbase and did not hit sections of the base where the Russians were believed to be present, Pentagon spokesman Captain Jeff Davis.

GETTY IMAGES

A survivor of the Assad regime's suspected chemical attack in Khan Shaykhun town of Idlib district, receives treatment.

Davis, briefing reporters on the operation, said the US military had "multiple" conversations with Russian forces before the strike, using a line of communication that had previously been established to prevent an accidental clash in Syria during the fight against Islamic State.

Earlier, Turkish President Recep Tayyip Erdogan had said he hoped Trump would take military action, Turkey's state-run Anadolu Agency quoted. Erdogan said Turkey would be prepared to do "whatever falls on us" to support possible military action, the news agency reported.

SYRIAN RESPONSE

A suspected chemical attack took place in Syria this week.

Syrian state TV says a US missile attack hit a number of military targets inside the country, calling the attack an "aggression."

A military official speaking on Syrian TV says an air base in central Syria was hit early Friday, causing material damage. The unnamed official did not elaborate.

Talal Barazi, the governor of Homs province, where the targeted air base is located, said most of the strikes appeared to target the province in central Syria.

He also said the strikes are meant to "support the terrorists on the ground."

AUSTRALIA BRIEFED

Syrian President Bashar al-Assad is thought to be behind the suspected chemical attack.

Australian Prime Minister Malcolm Turnbull hinted at having had briefings of the impending attack when asked about the Syrian conflict on Friday morning.

"We have been in close touch with our American allies on this, and I can't go into any more detail than that, other than to say that I've spoken only a little while ago with the defence minister and the Chief of the Defence Force," Mr Turnbull told 3AW. "We are in close touch with our American allies in that theatre."

In New Zealand, Foreign Minister Murray McCully reiterated the country's horror at the chemical attacks. 

"It is becoming clear that Syrian government forces were responsible for the outrageous attacks where chemical weapons were used," he said.

"These events are horrific. It is critical that the international community emphatically demand an end to this violence, and that the Syrian government be held to account."

"In the absence of an adequate response from the United Nations Security Council, we can understand why the United States has taken targeted unilateral action to try and prevent further such attacks by the Syrian regime."

INTENSE DELIBERATIONS

The decision to strike follows 48 hours of intense deliberations by US officials, and represents a significant break with the previous administration's reluctance to wade militarily into the Syrian civil war and shift any focus from the campaign against the Islamic State, the Washington Post reports.

Senior White House officials met on the issue of Syria on Wednesday evening (US time) in a session that lasted into early Thursday, and Defence Secretary Jim Mattis, Secretary of State Rex Tillerson and Army Lieutenant General H.R. McMaster, the national security adviser, have communicated repeatedly since the chemical attack, the officials said.

The US Central Command has had plans for striking the Syrian government for years and currently has significant assets in the region, enabling a quick response once a decision was made.

While the Obama White House began operations against the Islamic State in 2014, it backed away from a planned assault on Syrian government sites a year earlier after a similar chemical attack on Syrian civilians.

This week's apparent nerve gas attack in northern Idlib, with its widely circulated images of lifeless children, appears to have galvanised Trump and some of his top advisers to harden their position against the Syrian leader.

UN RESPONSE

US officials had said they hoped for a vote on Thursday night (US time) on a UN Security Council resolution that would condemn the chemical attack, but with council members still negotiating the text into the evening, the British Mission's political coordinator Stephen Hickey tweeted the vote wouldn't take place until later.

At the UN, the United States, which currently holds the presidency of the Security Council, it drafted a resolution along with Britain and France that condemns the use of chemical weapons, particularly in the attack on Khan Sheikhoun, "in the strongest terms."

Russia objected to key provisions in the resolution and negotiations have been underway to try to bridge the differences. Britain's deputy ambassador Peter Wilson said "what we want is a unanimous resolution ... and we want to see this done soon."

A day earlier, Russia had argued against holding Assad's government responsible.

France's UN Ambassador Francois Delattre indicated difficulty in reaching agreement on a resolution.

"We have engaged into negotiations in good faith to adopt a resolution - but make no mistake about it we need a robust text," he said. "We cannot be willing to have a text at any cost."

- AP, Reuters, Washington Post